Clearing the block might not fix everything if the draft is off. Sometimes cold air gets trapped or the flue is too big for the fireplace. Have you checked the damper position or looked for leaks in the chimney liner? What kind of fireplace do you have?
Yeah, clearing it can work like susan11 said, but what if the block isn't the main problem? I've seen cases where the chimney just doesn't pull air right because the house is too sealed up now. Have you tried lighting a small piece of newspaper up near the damper first to see if the smoke goes up? That tells you if the draft is actually working before you bother with the whole block.